In conventional peroxide brightening of mechanical pulp a brightening liquor containing perhydroxyl ions is first formed by reacting hydrogen peroxide with sodium hydroxide to form the perhydroxyl ion in water. This liquor containing the brightening chemical (perhydroxyl ion) is then added to the pulp and functions to oxidize color compounds in the pulp to colorless form thereby increasing the pulp whiteness or brightness.
The ratio of peroxide to caustic (sodium hydroxide) in the brightening solution must be optimized for each pulp. Too little caustic would leave some of the hydrogen peroxide in unreacted form while too much caustic causes the perhydroxyl ions to decompose thus wasting some of the perhydroxyl ion and further could result in alkali darkening of the pulp. Compounds are produced during the pulp brightening that tend to utilize the sodium hydroxide so that the optimum caustic to peroxide ratio varies depending on species and quality of the wood being brightened.
Generally, the magnesium sulfite and sodium silicate are incorporated in the brightening liquor, i.e. liquor containing the perhydroxyl ions to protect the perhydroxyl ions from decomposition until it reacts with the pulp, i.e. in storage and until the actual brightening reaction takes place in the pulp.
In a typical composition, magnesium sulfate and sodium silicate are first added in amounts related to the bone dry weight of the pulp being brightened. Peroxide addition is keyed to obtain the desired degree of pulp brightness thus is based on desired degree pulp brightness and on amount of pulp while the sodium hydroxide usage is dependent on the peroxide used (pulp brightness desired) and obviously as above indicated on the species being treated.
Because of the reactivity of the perhydroxyl ion the brightening solution has to be made by specific sequential additions of the individual chemicals. Normally, a cascading system is used wherein magnesium sulfate is first added to water and then sodium silicate is added and intimately mixed followed by the addition of caustic and finally by the addition of hydrogen peroxide, the resulting liquor then is stored in a bleach liquor storage tank until used.
This process obviously results in some decomposition of the reactive perhydroxyl ions in storage and requires a significant amount of process equipment such as tanks, pumps, mixers, etc for each of the separate ingredients. Also, the magnesium and silicate present in the mixture form complexes which precipitate and sometimes result in scaling and plugging problems.
It will be apparent that if there is a rapid change in wood quality it is not a simple matter of varying the brightening solution usage on wood, rather it is necessary to change the whole composition of the brightening solution. Clearly, when such changes are made they are not reflected in the system until the previously made brightening solution has been used thereby causing an inherent time delay when a change in the brightening composition for the pulp is implemented.
Canadian Pat. No. 686,115 issued May 12, 1964 to Guard describes a system of bleaching wood pulp wherein the wood is first treated wtih a chelating agent to remove or deactivate catalytic ions followed by thickening to typically about 14%-15%. Initially the resultant thickened pulp is treated with hydrogen peroxide followed by the addition of a buffered sodium hydroxide, i.e. sodium hydroxide and silicate solution, thereby silicates are incorporated into the system. This two stage bleaching technique is claimed to improved the brightness relative to conventional process, however, it is believed in this case the improvement in brightness was probably due to preliminary treatment with a chelate followed by thickening with possibly some improvement being attributed to the two stage process.
When using hydrogen peroxide as a bleaching agent it has normally been found that to obtain a given degree of brightness for a given pulp requires a specific percentage of hydrogen peroxide and that variations in the process usually do not materially change the total application of peroxide to pulp to obtain the given degree of brightness.
It is also kwown that there is residual peroxide in the bleaching liquor accompanying the bleached pulp from the bleaching tower, however, this residual generally does not exceed about 30% of the added peroxide thereby rendering the recovery of this residual chemical unjustifiable.